Method of cracking oil in liquid phase



July 9, 1929.

J. C. BLACK METHOD OF CRACKING OIL IN LIQUID PHASE Filed Oct. 26 1926INVENTOR ATTM 1,720,070 PATENT OFFICE.

JOHN C. BLACK, OF DESTREHAN, LOUISIANA.

METHOD OF CRACKING OIL IN LIQUID PHASE.

Application filed October 26, 1926. Serial No. 144,347.

This invention is directed to a process of cracking oil, particularly toa process of cracking oil in the liquid phase.

In my Patent $51,456,419 is described a process of cracking oil whereinoil is passed through coils in a furnace and subjected to a crackingheat for a prolonged period of time, .while maintaining the oil underpressure sufficient to overcome, substantially, the

vapor pressure of the oil.

It-is an object of this invention to devise an improved process wherebythe oil may be subjected to said cracking heat for a. prolonged periodof time, and to increase the period of time during which the oil issubjected to the aforementioned cracking heat.

It is well known that the cracking reaction is a function of both timeand temperature. At any given temperature the amount of oil decomposedis proportional to the time during which the oil is kept at thiscracking temperature. A greater decomposition is obtained for a givenperiod of time byincreasing the temperature at which the oil ismaintained, providing, of course, that the temperature is above theminimum cracking temperature of the oil.

It has been recognized that a process as described in my aforementionedpatent wherein the oil is heated in liquid phase at a crackingtemperature, allows for more efficient transfer of heat to the oil. Thisis so because none of the heat is absorbed as latent heat ofvaporization of the oil, all of the absorbed heat going to endothermicheat of the cracking reaction. Again, the coefficient of heat transferis smaller for liq uid oil than for vaporous oil, and the specific heatof liquid oil is greater than vaporous oil.

I have found also, that in addition to the above advantages thecharacter of the gas0- line formed by this liquid phase cracking isentirely different and of a better grade than that obtained by either avaporous phase, or combined liquid and vaporous phase cracking method.

An additonal advantage of this liquid phase cracking resides in the factthat a greater degree of conversion is obtainable since the oil can bemaintained in the cracking coil for a longer period of time, since themass velocity is smaller than in combined liquid and vapor phasecracking in tubes, or in vapor phase cracking. By operating at thisgreater period of time 1 may either get an increased decomposition atany given temperature, or by lowering the temperature and increasing thelength of time I may get the same conversion as would be obtained byoperating at a higher temperature and a shorter period of time. Theadvantage which resides in operating at lower temperature and increasedlengths of time arises from the fact that at lower temperatures a muchsmaller amount of carbon is deposited than is deposited at highertemperatures.

There is, however, a limit to the length of time and to the temperatureat which an oil may be subjected to such liquid phase cracking systems.The oil, under an imposed mechanical pressure, may be decomposed up to apoint where the generated vapor pressure of the cracked oil at thetemperature employed, exceeds the imposed mechanical pressure upon theoil. Any further decomposition would cause a vaporization of the oil inthe tubes. The operation would then be liquid phase, through the firstportion of the tubes,'until the point is reached, in the progressthrough the tubes, where the concentration of the light hydrocarbons inthe oil is such, that at the temperature to which the oil is subjected,their vapor tension overcomes this super-imposed pressure. From thatpoint on the operation is that of a combined vaporous and liquid phase,and may even, at a further point, become vapor phase cracking. Theresult of this is a deposition of carbon in the tubes, a great increasein the production of permanent gases, and decrease in the heatefficiency of the process, and a variation in the character of thegasoline obtained usually showing an increase in the specific gravity ofthe gasoline, although the gasoline may not be of decreased volatilityand also an increase in the olefine content of the gasoline.

The deposition of carbon results both from carbon of decomposition andalso is a result of the concentration of high boiling, tarlike bodies inthe oil, after a part of the oil has become vaporized. These tars settleout upon the tube surfaces and become carbonized rapidly, the tubesbecoming overheated, whereupon further skin cracking results. Thisaccelerates, and a very harmful operation is soon reached where gassingresults and tubes burn out.

It is an object of this invention to prevent this increase in vaportension of the oil to a point where it exceeds the imposed mechanicalpressure upon the oil, while yet allowlng the accumulation of theselight hydrocarbons and a further decomposition of the heavierconstituents of the partially cracked oil. The process in generalcomprises, heating oil under an imposed mechanical pressure at a chosencracking temperature and for a period of time, to a point where thecracked oil has avapor pressure approximating the imposed mechanicalpres sure; diluting the oil at this point and continuing the cracking ofthe diluted oil at the said imposed mechanical pressure for a furtherperiod of time. i

The invention will be better understood by reference to the accompanyingdrawing which shows a schematic arrangement of ap paratus forcarryingout the process of this invention.

1 is a tank containing the oil to be cracked. 2is a line connecting thistank with pump 3.

2 is a valve in said line. 4 is a line connecting pump 3 with heatexchanger A. 5 is a cross- :5 over line connecting heat exchanger A withheat exchanger B. '6 is a line connecting heat exchanger B with heatexchanger C. 7 is a line connecting heat exchanger C with coil 8situated in furnace 9. 10 is a cross-over line 1 connecting coil 8 withcoil 11 situated in furnace 12. 13 is an exit line connecting coil 11with heat exchanger C. 14 is a line connecting heat exchanger C withevaporator 15. 14' is a valve in said line. 16 is afractionating tower,such as a bubble tower, connecting with evaporator 15. 17 are bubbletrays 'g-i'n'said tower. 18 is a condensate trap in tower 16. 19 is ananalyzer for refluxing condensate. 20' is a line connecting tower 16with condenser 21. 22 is a lookbox in condensate run-down line 23 whichconnects to tank 24. 25'is a gas vent connected to lookbox 22. 26 is aline connecting the trap 18 with the upper trays 28 in still 27 29 is avapor line connecting still 27 with condenser 30- 31 is .a gas ventconnected to lookbox 32 in condensate run-down line 33 which lineconnects with ta k 34. 35 is aline connecting tank 34 with pump 36. 37is a line connecting pump 36 with the upper trays 17 in tower'16. 38 aresteam coils in still 27. 39

is a line connecting the liquid portion of still 27 with tank 42, viaheat exchanger A, line 40 and pump 41. 43 is a line connecting tank 42with intake side of pum 3. 43 is a valve in said line. 44in which t ereis a valve 44 is a line connecting evaporator 15 with heat exchanger B.45 is aline connecting heat exchanger B with pump 46. 47 is a lineconnecting pump 46 with tank 48. 49 is a bypass around exchanger Cconnecting lines 6 and 7. 49 is a valve in said by-pass. 50 is a by-passline connecting the exit side of pump 3 with coil 11 situated in furnace12. 50 is a valve in said line. Coil 11 is connected to line 50 bymeansof a series of crossover connections 51, 52, and 53 in which are anumber of valves 51", 52 and '53. Pyrometers 55, 56, 57, 58, 59, and 60are situated on either side of the point of connection of thesecross-overs with the coil. 54 is a pyrometer at the outlet of coil 8. 61is a pyrometer at the outlet of coil 11. 62 is a pressure gauge placedimmediately before the pressure reducing valve 14.

The operation of this system is as follows: Oil contained in tank 1 ispumped by means of pump 3 through'line 4, heat exchangers A, B, and C tocoil 8. In this coil the oil is raised to the desired crackingtemperature which is measured by pyrometer 54. It then passes throughcoil 11 where it is maintained at, at least, said cracking temperaturefor a prolonged period of time. Instead of raising the oil to thecracking temperature at the exit of coil 8 it may be raised to thiscracking temperature part way through this coil and maintained at thecracking temperature through the rest of the coil and through coil 11.The oil then passes through heat exchanger C where it is partiallycooled and the cooling regulated by regulating the amount of oil passingthrough heat exchanger C by the proper manipulation of valve 49, inby-pass 49. The oil then passes from heat exchanger C to evaporator 15,past-the pressure reducing valve 14'. The pressure of the oil during theheating step is regulated by the regulation of this valve 14. The oil oninjection in evaporator 15 is distilled, the unvaporized fraction beingremoved through line 44 and valve 44 and through heat exchanger B, andsent to storage tank 48 by pump 46. The vapors rising from 15 arefractionated in tower 16 by a proper regulation of the analyzer 19 andby the introduc-' tion of reflux oil through 37, as will appear below.The condensate trapped out in 18 I which consists of fractions heavierthan gasoline, still contains gasoline fractions, and is sent via line26 to still 27 where the hot c011- densate is distilled andfractionatedby aid of steam 38 and bubble trays 28. The vapor issuing from 29consists of the heavy gasoline fractions. These are condensedincondenser 30, collected through lookbox 32 and line 33 in tank 34.Uncondensed gases are removed via line 31. This condensate in tank 34 issent up via pump 36 and line 37 to act as the reflux aforementioned. Theunvaporized portion remaining in still 27 is removed via line 39 andvalve 39 through heat exchanger A in line 40 and stored in 42, by meansof the pump 41. This condensate may be Stored for future cracking athigher temperature, or may be recycled to mingle with the oil removedfrom tank 1, by

, manipulation of valve 43 in line 43. Vapors issuing from the tower 16which consist of either pressure distillate or crude gasoline,

are condensed in condenser 21, collected via lookbox 22 in line 23 intotank 24. Unvaporized gases uncondensed in condenser 21 are removed vialine 25.

The temperatures and time during which the oil is heated aredeliberately chosen so that the oil will reach a vapor pressureexceeding the imposed pressure on the oil. Therefore, at some point inits progress through the coil,

for instance, in coil 11, the oil will have reached a partiallyvaporized condition. When this happens, a pyrometer situated in the oilstream will begin to fluctuate. This is due to the fact that thispyrometer is in the region where it is at one moment subjected to anatmosphere of vapor, and, at another moment, to an atmosphere of liquidoil. The heat transfer from vapor to the pyromcter is not as good asfrom liquid oil, and the pyrometer begins to fluctuate up and down. Allpyrometers situated in-the coils from this point to the outlet will showthe same effects, or will show a falling tem perature. Supposing thiseffect is first noted on pyrometer 55. Oil is injected at this point vialine and cross-over 53, valve 53' eing open, and valves 51 and 52 beingclosed. It will then appear that pyrometers 56, 57, 58, 59 and 60 becomesteady due to the addition of relatively cool and uncracked oil whichacts as a solvent for the low boiling hydrocarbons, and thus reduces thevapor pressure of the oil to a point where it does not exceed theimposed mechanical pressure. The oil then passes through the remainingportion of the coil where it is raised back to its cracking temperature,and the cracking is continued for an increased length of time. Should itappear that after this injection, that whereas pyrometers 55 and 56 aresteady, 57 has begun to fluctuate, as also have 58, 59, and 60, oil maybe injected via cross-over line 52 to reconvert the partially vaporizedoil stream back to a liquid phase. The pyrometers then become steadyagain. By so manipulating the oil stream and injecting cool oil, orrelatively cool and uncracked oil to dilute the oil at the point wherethe partial vaporization of the oil occurs, the oil may be maintained inliquid phase throughout its course through the coils, and subjected fora greater length of time to cracking in the liquid phase at the chosencracking temperature. The injected oil may be relatively cool oil or maybe preheated to the cracking temperature aforementioned and thisrelatively uncracked oil injected into the stream to dilute the morecracked oil and so reduce its vapor pressure.

" Instead of injecting the oil at the point where the pyrometers show afluctuation where the cracked oil has reached a vapor presure such as tocause vaporization, the oil ma. be injected at a point in front of thisp ace, that is at an earlier point. This will have the same effect andif the subsequent heating is not too prolonged no vaporization willoccur. Likewise the injection may be made at a place after the point ofincipient vaporization to obtain a slight degree of vapor phase crackingif the advantage of vapor phase cracking is desired. This amount ofvapor phase cracking must, however, be strictly limited to prevent theaccumulation of carbon and excessive permanent gas formation.

The oil by means of this operation can therefore be maintained for agreater length of time at its reaction temperature, or the oil may bemaintained at a higher reaction temrature than would be possible if thisinjection of oil .were not employed. Having determined the position ofthe point of injection in the operation of the system for any given oil,time, and temperature, it will be no longer necessa to have all thecrossovers originally emp oyed. The system may be built only with thosecross-overs actually needed. N or is it necessary to start in with acoil containing a number ofcross-overs, such as 51, 52, and 53, since itwould be possible to weld into the coil any cross-overs which mightappear to be necessary on the inspection of the operation of thesepyrometers. It is usually undesirable to have a large number ofconnections or a large number of valves on a coil operating under thehigh pressures and temperatures usually employed in this system, whichpressures are in the neighborhood of one thousand pounds and thetemperature between approximately 800 F. and 900 F.'

The exact locations of these cross-over lines and the length of timeduring which the oil may be exposed to heat at the cracking temperature,will depend upon the character of the oil cracked and the temperatureand pressure employed. Thus, in cracking gas oil at a temperature intheneighborhood of 820 F. to 840 F. at pressure approximately 1,000 pounds,the oil may be subjected for approximately twenty minutes withoutsubstantial vaporization in the coils. If the heating is continuedbeyond this point for any considerable length of time, it will be foundthat oil must be injected into the coils. The length of time duringwhich the oil may then be heated without its flashing into vapor phasewill depend upon the amount and character of the oil injected. However,if one operates at a temperature in the neighbor hoodof 850 F. to 880 F.the length of time during which the oil may be subjected to heat at thistemperature will be materially less, sincethe rate of cracking reactionis considerably augmented and it ma be found that the injected oil mustbe intro uced after eight or ten minutes of cracking reaction.

The ides to determining the presence of vapor piiise cracking are asstated above, fluctuation of the pyrometers an increase in the amount ofcarbon and fixed gases formed, an

increase in the specific gravity and olefine content of the gasoline.The surest guide is the fluctuations in the pyrometers placed in thecoils, as will appear to those skilled in the art.

The above is not .to be taken as limiting my invention but merely asillustrative of the best manner of carrying out my invention, which Iclaim to be:

1. A. process of cracking oil which comprises heating oil in a confinedstream under superatmospheric pressure to crack said oil, measuring thetemperature of said oil by pyrometers at a plurality of places alongsaid stream, and injecting oil at points in said stream wherefluctuations in temperature, as indicated by said pyrometers,'areobserved.

2. A process of cracking oil which comprises heating oil to a crackingtemperature,

maintaining at least said cracking temperature while passing said oilunder superatmosplieric pressure in a confined stream, measuring thetemperature of said oil by pyrometers situated at a plurality of pointsalong said confined stream, and injecting relatively cool oil in theregion where fluctuations in temperature, as indicated by saidpyrometers, are observed.

3. A process of cracking oil which comprises heating 011 in a confinedstream under superatmospheric pressure to crack said oil,

measuring the temperature of said oil by pyrometers at a plurality ofplaces along said stream, and injecting relatively uncracked oil 1n theregion in said stream where fluctuation along said confined'stream andinjecting relatively uncracked oil in the region where fluctuatlons intemperature, as indicated by sald pyrometers, are observed.

5. A process of cracking oil which comprises heating oil in a confinedstream under superatmospheric pressure to crack said oil, measuring thetemperature of said oil by pyrometers at a plurality of places alongsaid stream, and injecting relatively cool and uncracked oil in theregion in said stream where fluctuations in temperature, as indicated bysaid pyrometers, are observed.

6. A process of cracking oil which comprises heating oil to a crackingtemperature, maintaining at least said cracking temperature whilepassing said oil under a superatmospheric pressure in a confined stream,measuring the temperature of said oil by pyrometers situated at aplurality of points along said confined stream, and in ecting relativelycool and u'ncracked oil in the region where fluctuations in temperature,as indicated by said pyrometers, are observed.

In testimony whereof I aflix my signature.

JOHN C. BLACK.

